Although wild mushrooms have been consumed for many centuries, commercial mushroom production is a relatively recent development in the history of agriculture. Commercial mushroom production uses composted vegetable materials as a substrate for supporting the growth of mushrooms, the substrate being prepared in as controlled a manner as possible in order to provide a growth medium favorable for colonization and growth of mushrooms despite the presence of a competing microorganism population. However, the solid-state fermentation composting process itself remains more of an art than a science.
Historically, composting began with straw containing manure obtained from horse stables. Because of the variable condition of stable bedding materials and weather, little advance was made in composting techniques until the beginning of this century. The realization that wheat straw was an important basic constituent as a carbon source and not, as many considered, necessary only for its physical effect on water-holding capacity and aeration was an important step in bringing composting procedures under control. Mushrooms were shown to be grown successfully on so-called synthetic composts prepared from mixtures containing wheat straw as the basic component with additional carbon sources such as corn cobs or brewers grain nitrogen sources such as ammonium nitrate, urea, and calcium cyanamide. The use of inorganic raw materials to replace the nutrients present in horse manure is a key feature in distinguishing a synthetic from a "natural" compost.
Currently there are no standard formulations for the commercial mushroom industry. However, most processes rely on either wheat straw (synthetic compost systems) and/or stable bedding (horse manure compost systems) as the matrix material for the growing substrate. In both systems, an initial preconditioning of the matrix occurs, although under different circumstances. This preconditioning phase is sometimes referred to as a prewetting phase. Horse manure systems rely on the mechanical breakage of straw under the animal's hooves combined with the biological activity that takes place on the stable floor to provide the first steps of breaking down the physical and chemical structure of straw. In the synthetic process, the physical degradation associated with the preconditioning step is carried out by shattering the straw by mechanical means, e.g., using bale-breakers, front-end loaders, and other mechanical devices. In the synthetic systems, after the straw is broken out of bales it is formed into large piles that are moved by large equipment such as front-end loaders. Water and a nitrogen source, typically urea, are added to the straw pile to provide the necessary moisture and nutrient conditions to stimulate microbial activity. Water is added because both bacteria and fungi are not active below approximately 10-13% moisture content. Urea is added because the nitrogen in straw is present in small amounts and is not readily available for biological comsumption. The straw provides the carbon nutrient component in the form of cellulose, hemicelluloses, and waxes. To maintain aerobic fermentation and uniformity, the large piles are turned and moved, usually every other day while adjusting moisture.
During this preconditioning fermentation, microorganisms degrade the straw by virture of utilizing it as a carobn source. Microbes secrete enzymes that break down celluloses, waxes, and other plant parts into simpler biomolecules such as sugars and fatty acids that are assimilated and readily utilized by the microorganisms. The initial enzymatic degradation results in a softer, more pliable straw that is ready for the next step of the fermentation.
Another result of the fermentation in large piles (typically 10 or more feet high and wide and 40 feet or more in length) is the production of heat as a metabolic byproduct along with carbon dioxide and water. The heat, in combination with the insulating nature of the large pile, produces high core temperatures. These temperatures cause non-biological caramelization of carbohydrates and sugars.
Although this initial fermentation prepares the straw for further composting and has been considered essential for preparing a compost that enhances mushroom production, the process is not particularly efficient because of the enormous amount of energy and biomass lost during the process. Typically, 20-30% of the straw is lost as carbon dioxide and water during this initial preconditioning fermentation. This represents a significant portion of the operation's variable cost of production. In some large growing operations, straw costs alone can be several million dollars per year.
In horse manure compost systems, the preconditioning fermentation can be shorter because of the initial breakdown of the straw on the stable floor. However, some initial preconditioning generally is used.
Once preconditioning is complete, the partially composted substrate enters into a stage known as Phase I composting. Because of the standardization of the mechanized equipment used in commercial mushroom production and physical restrictions resulting from the need to retain heat while providing for aerobic fermentation, Phase I composting typically takes place in 6.times.6' windrows known as ricks that are turned by specially designed heavy equipment to maintain an aerobic, thermophilic fermentation while providing the means to add supplements and water. The supplements typically comprise a wide variety of protein and/or carbohydrate-rich materials which, with water added concurrently, aid in the continued fermentation of the compost. This second fermentation further decomposes the straw and in the process also degrades the supplements added to fuel the process. The objective of this stage of the fermentation is to moderate the fermentation process so that at the end of Phase I mushroom growth will predominate over competing microorganisms in the compost. To accomplish this, the Phase I fermentation is designed to utilize the soluble carbohydrates and proteins present that would readily be available to competing microbes once the compost is spawned with the mushroom. As indications of the depletion of nutrients available to competing microbes, growers typically look for a drop in temperature, straw that is increasingly pliable, and straw caramelization. At the completion of Phase I, the biologically modified straw, supplements, and inactive microbial biomass provide the selective food base on which the mushrooms will later grow.
To finish the composting process, the compost must be pasturized to prevent excessive microbial growth and further must be purged of free ammonia (which is toxic to mushrooms) produced by the degradation by bacteria of nitrogen-containing supplements. This step is also a solid-state fermentation, which is referred to as Phase II. Unlike Phase I, this step is typically accomplished in a specially designed room where environmental conditions can be controlled. Phase I fermentation typically occurs outside either on open concrete pads or on concrete pads with roofs. In Phase II, the addition of external heat initiates compost thermogensis. Residual nitrogen is either ammonified and driven off or converted into microbial protein. Any residual sugars, fats, and other small molecules are also used by the mostly microbial biomass. At the beginning or (rarely) end of the process, there is a step referred to as peak heating. Heat is introduced into the room to provide temperatures in the compost of about 60.degree. C. for a period of time to accomplish pasturization. This step eliminates insects, nematodes, and competitive fungi that would be detrimental to the mushroom crop. Once Phase II is completed, the compost is ready to be spawned with the mushroom, thus ending the composting process.
There are a number of undesirable features of the prior art composting process as described above. The initial composting typically relies on envionmental microorganisms to carry out the initial fermentation and therefore can be quite variable. It would be desirable to inoculate straw with a defined bacterial population to provide for optimal initial fermentation and uniform results. However, this is difficult because of the presence of environmental organisms. Accordingly, it is difficult to create an assembly line-like process for the preparation of compost, which would reduce production costs.
Additional production costs arise from the degradation and loss of composting material, which can be minimized if the composting time is short. Shortening the composting time also reduces labor and equipment costs required by the constant turning of the large fermentation stacks. However, any shortening of the compost time must be carefully reviewed in view of the product produced. If the fermentation does not consistently produce a substrate suitable for mushroom growth, it will be worthless. Accordingly, if a suitable substrate could be obtained by a shorter fermentation process, the savings in labor, space, and equipment costs would be substantial.